1. Field of the Invention
The present invention relates to an optical device, an optical system a method of production of the optical device, and a mold for production of the optical device.
2. Description of the Related Art
When producing a lens, the following first to third methods of production have been known.
The first method of production is a method of filling an optical material such as molten glass in a metallic mold formed with a cavity of a desired lens shape machined to an intended lens shape to produce a mold lens.
The second method of production is a method of utilizing reactive ion etching (RIE) or other etching and using a photo resist or the like as a mask (etching mask) to etch an optical material to a predetermined shape to thereby produce a lens made of the related optical material.
The third method of production is a method of mechanically polishing a base made of an optical material to the lens shape to produce the lens.
In the conventional first method of production, that is, the method using simple molding, it is difficult to produce a small sized lens having a large numerical aperture (NA), so it is difficult to reduce the lens diameter to 1 mm or less.
In the conventional second method of production, that is, the method using RIE or other etching, there is a problem in that there are restrictions on the optical material and there are a few optical materials of high refractive indexes capable of obtaining lenses having large numerical apertures among the optical materials capable of RIE and other etching, so it is difficult to use a material having a high refractive index and it is difficult to realize a lens having a large numerical aperture NA.
In the conventional third method of production, it is difficult to manufacture a small sized lens.
If increasing the numerical aperture of the lens, it is possible to make the size of a light spot created after passing through the lens small. It is desirable from the viewpoint of increase of the capacity of an optical disc to enlarge the numerical aperture NA of the lens (object lens) of an optical head.
Also, lenses and other optical devices are being used for various optical apparatuses. Reduction of the size of the optical devices is desirable from the viewpoint of the reduction of size of the optical apparatuses such as an optical disc apparatus and an optical pickup.
In order to realize an optical device having a large numerical aperture, a large refractive index of the optical material is effective.
As an optical material having a high refractive index in a region of visible light, there are titanium oxide, tantalum oxide, gallium phosphate (gallium phosphorus), gallium nitride, silicon nitride, etc.
However, it is difficult to machine these materials to small sized lenses having a large numerical aperture in the prior art.
Also, many conventional lenses have irregular shapes. In order to align a plurality of lenses of such irregular shapes, high precision positioning in three-dimensional directions is necessary, so the load of the alignment work is large.
Also, when comprising a flying head (floating head) consisting of an optical head mounted on a swing arm, the optical head can be prepared by separately preparing a slider and the lens and attaching them at a high precision, but in this case, the load of the attachment work and accordingly the load of preparation of the optical head is large.